The SMUGGLE-Ring project: Bar and bulge effects on nuclear disk and ring formation

Abstract

We present the first results from the SMUGGLE-Ring project, a suite of simulations employing the SMUGGLE ISM and stellar feedback model to explore nuclear structures in Milky Way-mass galaxies. We discuss results from three simulations evolved for 5 Gyr in isolation, in which we vary the classical bulge mass, while keeping the disk and halo structures identical. Nuclear stellar disks and rings emerge exclusively in our bulge models, with more massive bulges associated with earlier formation and more extended initial gas reservoirs shortly after bar formation. After gas depletion via active star formation, the nuclear stellar disks bifurcate into pressure-supported nuclear star clusters (NSCs, $v_φ/σ_R < 0.7$) and rotationally supported nuclear stellar rings (NSRs, $v_φ/σ_R = 1.2$--1.7, radii 0.64--0.76 kpc). The bulgeless model fails to build up and sustain stable nuclear gas disks against feedback disruptions. The enclosed stellar mass of NSCs ($\sim10^{9}\Msun$) dominates over that of NSRs ($\sim10^{8}\Msun$). The star formation rates decline over time due to gas depletion (NSCs 0.1--1 $\Msun$/yr, NSRs 0.01--$0.1 \Msun$/yr). Kinematics reveal outward-shifting rotation peaks with $σ$-drops in NSRs, while a fraction of stars in NSCs exhibits radial shift after 3 Gyr. These findings support inside-out NSD formation via secular bar evolution, with NSRs tracing the star-forming outer edge of the nuclear gas disk and NSCs forming the kinematically hotter inner component. The range of nuclear stellar disk sizes (0.25--0.76 kpc) falls within the observationally inferred ranges, but the existence of larger rings would require external gas flow and/or a longer period of evolution. Future SMUGGLE-Ring extensions will incorporate varying gas fractions, tidal/merger effects, and the circumgalactic medium to further elucidate nuclear diversity and outliers.

Figures (17)

• Figure 1: Distribution of the Fourier mode $m=2$ within 8 ${\rm\,kpc}$, illustrated by the time evolution of the radial Fourier distributions for each model. The time interval between snapshots is 0.01 Gyr. The color bar scale is fixed to range from $1\%$ to $15\%$ across all models. Only the initial stellar disk particles are selected to calculate the Fourier mode $m=2$, after excluding the initial classical bulge component. The radial profiles of the $m=2$ modes at representative times are presented in Figure \ref{['fig:f2radial']}.
• Figure 2: Face-on projections of the stacked surface density distributions of stars and gas within a $15 \times 15\,{\rm\,kpc}^{2}$ box at 2.5 Gyr for the models r1c14b00, r1c14b05, and r1c14b10 (from left to right). The yellowish-brown colors represent the stellar distribution, while the dark regions indicate the gas distribution. Overlaid blue and purple points mark young stars with ages $0.01 < t_\mathrm{age} < 0.1 \, {\rm\,Gyr}$ and $t_\mathrm{age} \leq 0.01 \, {\rm\,Gyr}$, respectively. The same projections at 1, 2, 3, 4, and 5 Gyr are shown in Fig. \ref{['fig:face_stack']}. The separate surface density distributions of stars and gas are presented in Fig. \ref{['fig:face_stacking']}.
• Figure 3: Face-on projections of the gas surface density distributions within a $15 \times 15$ kpc box at 1, 2, 3, 4, and 5 Gyr (top to bottom rows) for models r1c14b00, r1c14b05, and r1c14b10 (left to right columns). The rightmost column shows zoom-in views of model r1c14b10 within a $6 \times 6$ kpc box at the corresponding times. The top and bottom color bars indicate units of ${\rm\,M_\odot}$ kpc$^{-2}$ and apply to the main gas density distributions and the zoom-in views, respectively.
• Figure 4: Temporal evolution of nuclear ring properties, enclosed masses of specified components within different radial extents. Model names are labeled inside the panel. Grey, red, and blue colors indicate models r1c14b00, r1c14b05, and r1c14b10. Model r1c14b00 does not form a long-lived nuclear ring. The top-left panel shows the ring radius as a function of time, determined by the mass-weighted mean radius of young stars. Using this radius, ellipse fitting is applied to measure the semi-major (upper line) and minor (lower line) axes of the nuclear ring over time in the top-middle panel, while the ellipticity is shown in the top-right panel. The bottom-left panel displays the enclosed mass within 1 kpc for the three models, including newly formed stars (dotted lines), gas (dashed lines), and their combined mass (solid lines). In the bottom-middle and bottom-right panels, the enclosed masses of these new stars between the semi-major and minor axes (nuclear ring) and within the semi-minor axis (nuclear star cluster) are plotted.
• Figure 5: Star formation rates for the three models, both total and within $R < 1$ kpc. The corresponding SFR for each region is labeled with different line styles, where $a$ and $b$ indicate the semi-major and semi-minor axes of the nuclear ring, respectively.